K562 cells are used as a model to study the control of human globin genes expression. They express embryonic and fetal but not adult globin genes. Results suggests that the lack of transcription of the adult beta-globin gene results from the absence/presence of an activating/repressing trans-acting activator or a repressor. This is being tested by studying the transient expression of the bacterial CAT enzyme in K562 cells transfected with plasmids in which the transcription of the CAT gene is driven by various globin gene promoters. In competition experiments, cells are co-transfected with the same plasmids and an excess of the corresponding globin promoter. Similar experiments are conducted with K562 cells grown in the presence of 5-azacytidine, which has been previously shown to induce beta-globin gene transcription in these cells. This represents a necessary step toward further characterization and isolation of this trans-acting factor(s).